Magnetic fields

Question 1

What is the right hand rule and what does it show?

Answer 1

Do a thumbs up with your right hand. If current is traveling up (the way the thumb is pointing) then the magnetic field will travel anticlockwise; the direction your fingers point.

Question 2

What does each finger represent in Fleming’s left hand rule?

Answer 2

Thumb = Motion (force)
Index finger = Magnetic field N to S
Middle finger = Current (+ to -)

Question 3

What is magnetic flux density, B, defined as?

Answer 3

The force on one metre of wire carrying a current of one amp at right angles to the magnetic field.

Question 4

What is the unit of magnetic flux density?

Answer 4

Tesla, T.

Question 5

Is Flux density a vector or a scalar quantity?

Answer 5

Vector quantity.

Question 6

State a practical which can be used to measure flux density.

Answer 6

Using a top pan balance, some magnets, some wires and a variable resistor. Adjust the scale with the magnets and wires on when there is no current to calibrate the scale. Then turn on the power and adjust the resistor by even increments and measure the mass. Take lots of readings then use F = BIl to calculate the flux density.

Question 7

When a charge enters a perpendicular magnetic field, which direction does the force act on it and how will it move due to this?

Answer 7

The force acts perpendicularly to the direction of travel causing circular motion.

Question 8

How do you use Fleming’s left hand rule for charged particles in magnetic fields?

Answer 8

Replace current with the direction of motion of the particle and keep it the same if it’s positive but swap the direction of the index finger and the middle finger if it’s negative.

Question 9

Force on a charged particle in a magnetic field is independent of what?

Answer 9

It’s mass.

Question 10

How does a cyclotron work?

Answer 10

Charged particles are fired into one of the electrodes, the magnetic field makes them follow a semi-circular path and then they leave the half across a gap which has an applied potential difference across it so the particles speed up slightly causing the radius to increase. This is caused by the pd being reversed to accelerate the particles.

Question 11

What is magnetic flux density?

Answer 11

The strength of the magnetic field; how close the field lines are together.

Question 12

What is electromagnetic induction?

Answer 12

When an emf is induced in a conducting rod by relative motion in a magnetic field.

Question 13

What does adding more coils of wire do to the emf?

Answer 13

A bigger emf will be induced.

Question 14

Why does increasing the number of turns increase the emf?

Answer 14

There is more current cutting the field lines.

Question 15

What is the unit of flux linkage?

Answer 15

Weber or Weber-turns.

Question 16

State a practical which can be used to investigate how the angle of a coil in a magnetic field can affect the induced emf.

Answer 16

Use a search coil inside a solenoid and a protractor next to it and connect it to an oscilloscope with the time base turned off. Record the emf from the amplitude for multiple angles at even increments of 10 degrees.

Question 17

What is Faraday’s law?

Answer 17

Induced emf is directly proportional to the rate of change of flux linkage.

Question 18

What does the gradient of a flux linkage against time graph give?

Answer 18

The size of the emf.

Question 19

What does the area under the graph of the magnitude of the emf against time give?

Answer 19

The flux linkage change.

Question 20

What is Lenz’s law?

Answer 20

The induced emf is always in such a direction as to oppose the change that caused it.

Question 21

Which equation do you use to find the magnitude of induced emf?

Answer 21

The given equation for magnitude of induced emf.

Question 22

Which equation do you use to find the induced emf.

Answer 22

The negative of the given equation for the magnitude of induced emf.

Question 23

How does an alternator work?

Answer 23

A coil rotates between two magnets with opposite poles, every half rotation the output voltage and current change direction producing alternating current.

Question 24

How does flux linkage vary in an alternator as the coil rotates?

Answer 24

Sinusoidally.

Question 25

How does induced emf in an alternator vary as the coil rotates?

Answer 25

Sinusoidally.

Question 26

What does an alternating current look like on an oscilloscope?

Answer 26

A sine curve.

Question 27

What does a direct current look like on an oscilloscope?

Answer 27

A horizontal line.

Question 28

What is the potential difference of the UK mains supply? What type of current is it?

Answer 28

230V alternating current, this is the rms value.

Question 29

What is a step-up transformer?

Answer 29

A core with more turns in the secondary coil than the primary coil to increase the pd.

Question 30

What is a step-down transformer?

Answer 30

A core with more turns in the primary coil than the secondary coil to decrease the pd.

Question 31

What is the main cause of energy loss in transformers?

Answer 31

Eddy currents.

Question 32

What are eddy currents?

Answer 32

Looping currents induced by the changing magnetic flux in the core. They create a magnetic field that acts against the field that induced them, reducing the field strength. They also dissipate energy by generating heat.

Question 33

How can the effects of eddy currents be reduced?

Answer 33

Laminating the core with layers of insulation.

Question 34

How can heat generated by resistance in the coils of a transformer be reduced?

Answer 34

Use thick copper wire which has low resistance.

Question 35

What is the use of transformers?

Answer 35

For stepping up the voltage when transmitting electricity across the national grid and stepping it back down again so it is safe for domestic use.

Question 36

What is the equation for power lost in a transformer?

Answer 36

P = I²R